Improved error reporting for assertions where there are no form error…

…s, or a non-existent field has been provided. Also created a test client regression test, to hold the tests validating the failure cases of the TestCase assertions.
git-svn-id: http://code.djangoproject.com/svn/django/trunk@5181 bcc190cf-cafb-0310-a4f2-bffc1f526a37